Optical fiber cables enable data to be transmitted under excellent conditions and over ever increasing distances.
Until now, use thereof has been mainly restricted to long-distance transmission, in inter-city and inter-exchange networks. At present, attention is being paid to using them in distribution networks, and even within buildings and residential areas, particularly in the context of developing integrated services digital networks (ISDN).
Unfortunately, conventional optical fiber cable structures are adapted to inter-city and inter-exchange networks and are more suitable for fibers in small to medium numbers, whereas an optical fiber cable for a distribution network needs to have a much larger number of optical fibers than is required for long distance transmission, since it is necessary to take account of a multiplicity of subscriber terminations (in practice, the number of optical fibers in such a cable may lie in the range a few hundreds to a few thousands), yet the cable must nevertheless be sufficiently compact to enable it to be installed in ducts of very small diameter.
Optical fiber cables presently in use for long distance transmission are of two types.
A first type of optical fiber cable is of a structure that is "cellular" or "free", i.e. it includes one or more elements in the form of a grooved rod or in the form of a tube in which optical fibers are housed. In order to reduce the bending and microbending to which the optical fibers are subjected, and which give rise to increases in attenuation, the optical fibers are left free and a certain amount of excess length is provided within the grooves or the tube.
It will readily be understood that in order to leave room to take up the excess lengths of the fibers, such a structure requires a very large ratio between the section of the cable as a whole and the sum of the sections of the individual optical fibers included therein. A structure of that kind is therefore quite unsuitable for use in a distribution network since cable sections would then be prohibitive, given the very large number of optical fibers that a cable must contain in order to be used in such a network.
A second type of optical fiber cable has a "ribbon" structure, i.e. the optical fibers are disposed parallel to one another and they are held together by a protective coating of resin. Cables of that type are more compact and less bulky, since there is no need to provide for excess lengths of fiber. Nevertheless, the ribbon structure is better adapted to connection of all the fibers simultaneously and that can give rise to problems in a distribution network. In a distribution network, it is necessary to provide for continuous evolution in the topology of the network, and that requires a ribbon to be split up, which operation is difficult to make compatible with the constraints of simultaneous connection since it does not provide sufficient flexibility.
Thus, it can be seen from presently existing optical fiber cable techniques that they are ill-suited to use within a distribution network.